The new series of American Express US dollar travellers cheques, first issued in 1997, employs as an anti-counterfeiting feature a diffraction grating foil image of the American Express Centurion logo. When illuminated by a light source and the diffraction grating foil device is observed from different viewing angles the Centurion image appears to switch to an American Express box logo image. This optical variability of the device ensures that it is impossible to copy by normal photocopier or camera techniques.
Diffraction grating devices which exhibit this variable optical behaviour are referred to as optically variable devices (OVDs) and their use as an anti-counterfeiting measure to protect valuable documents is continuing to grow. Examples of particular proprietary optically variable devices and applications to date include the EXELGRAM™ device used to protect the new series of Hungarian banknotes and American Express US dollar and Euro travellers cheques and the KINEGRAM™ device used to protect the current series of German and Swiss banknotes. The EXELGRAM™ device is described in U.S. Pat. Nos. 5,825,547 and 6,088,161 while the KINEGRAM™ device is described in European patents EP 330,738 and EP 105099.
The KINEGRAM™ and EXELGRAM™ devices are examples of foil based diffractive structures that have proven to be highly effective deterrents to the counterfeiting of official documents. This class of optically diffractive anti-counterfeiting devices also includes the PIXELGRAM™ device that is described in European patent number EP 0 490 923 B1 and U.S. Pat. No. 5,428,479. PIXELGRAM™ devices are manufactured by producing a counterpart diffractive structure wherein the greyness values of each pixel of an optically invariable image are mapped to corresponding small diffractive pixel regions on the PIXELGRAM™ device. In the PIXELGRAM™ device the greyness value of a pixel corresponds to the red (R), green (G) and blue (B) colour values of the pixel in the case when all three values are made equal (i.e. R=G=B).
In spite of their industrial effectiveness these foil based diffractive optically variable devices also represent relatively expensive solutions to the counterfeiting problem when compared to the more traditional security printing technologies such as watermarking and intaglio printing. The expensive nature of these technologies is due to the requirement for embossing the diffractive microstructure into a metallized plastic foil prior to the application of this foil onto the valuable document.
Because the embossing of the OVD microstructure takes place in a specialised foil production facility external to the security printing works there is also the added problem and potential security risk if the high security foil supplies are lost or stolen in transit to the security printing plant. For these reasons security printers would prefer to have access to an OVD technology in the form of a specialised printing die that did not need to be applied as a hot stamping foil and could instead be directly printed onto the valuable document using specialised inks or lacquers in line with the normal intaglio printing process.
International patent application PCT/AU99/00741 describes one approach to the problem of developing a three dimensional microstructure that can be directly embossed or printed onto a valuable document. In this application the method of manufacture of the device involves the contact printing of a transparent electron beam lithography generated greytone mask structure into a thick optical resist layer whereby the height of the exposed resist in a particular region of the image is directly related to the optical transparency of the greytone mask in that region and each pixel region of the greytone mask is mapped to a group of microstructure elements on the exposed resist surface. In the patent application PCT/AU99/00741 the structure of the greytone mask pixels is limited to arrays of transparent square apertures or transparent track elements of variable width and length within each pixel region.
This approach is able to generate relatively deep optical image microstructures when compared to diffractive devices and is an advance over previous greytone techniques based on single pixel masks such as in the paper by Reimer et al in “Proc. SPIE Vol 3226, Microelectronic structures and MEMS for Optical Processing III, Austin, Tex., 1997”. However the variability of the surface profile of the device, and therefore the consequent optical variability of any image generated by the device, is limited by the requirement of having only one pixel parameter (the greytone value) in the optically invariable image relate to the geometrical characteristics of the three dimensional microstructure. In particular this one parameter limitation means that only the height of the microstructure is able to be controlled within each small region of the microstructure.
The utility and applicability of the technology described in PCT/AU99/00741 is also further constrained by; (a) the requirement to limit the optical exposure geometry to a contact printing arrangement, (b) the requirement to limit the greytone mask pixel functions to arrays of transparent rectangular apertures or arrays of transparent track-like elements of variable width, (c) the need to have a significant number of high aspect ratio regions on the device and (d) the requirement to relate the transparency of each pixel region of the mask to the depth only of each corresponding pixel region on the device. Therefore both the geometrical surface characteristics and the method of manufacture of the device described in PCT/AU99/00741 are of limited utility in terms of industrial application.